Sorry for the long absence, the holidays impeded my progress. Upon
consultation with both a structural engineer and building contractor,
it turns out the angle braces were there to fix the cracks that had
developed in the joists near the tenons. When I jacked up the beam,
they came up but not as far as the beam and that was due to both
increased sagging as well as further cracking at the tenon joints.

The joists are hand-hewn and, interestingly, the joists spanning from
the beam to the back sill are significantly larger than those spanning
from the beam to the front. This problem afflicts the smaller joists
much more seriously and those will need replacement while the larger,
rear joists will need sistering.
Thanks for everyone's thoughts on this problem.